Contemporary communication system designers are continuously challenged with transmitting more and more information down a given communication channel. For example, the current standard for a land mobile communication channel comprises a radio frequency channel having a bandwidth of 25 kHz for one message. Conventionally, two such channels are required for a full duplex conversation: one to transmit, and one to receive. As available frequency spectrum diminishes, the need to transmit more information down a communication channel becomes paramount. This need is experienced today in urban areas where a limited amount of frequency spectrum must be shared by a large number of transceivers in a given geographic area.
One technique to send more information down a communication channel is time compression multiplexing (TCM). A TCM communication system comprises an analog system wherein analog signals are sampled and stored in a storage means. The samples are extracted, in turn, and transmitted at a high rate of speed. In this way, several signals may be sent over the same communication channel in a time division multiplex (TDM) fashion. In a TDM system, a communication channel is divided into a plurality of slots. Each transceiving device may transmit or receive information in one or more of the slots.
TCM advantageously exploits the fact that time compression merely scales the occupied spectrum in relation to a time scaling factor. Accordingly, two voices compressed by approximately 2:1 may be transmitted over a single conventional 25 kHz radio channel by slightly reducing the deviation, and improving filtering to reduce adjacent channel "splatter". TCM stands in contrast to digital coding techniques that require considerably more complexity and processing for a speech signal to occupy a given bandwidth.
The TCM slot duration (i.e., the duration of the speech burst), is selected by balancing several considerations, including audio delay (preferably small), and the limitations of adapting a transceiving device from the transmit to the receive mode, or vice versa (i.e., synthesizer and antenna switch settling times). A typical value for a 2:1 compression system comprises a 60 ms slot. A full duplex system is synchronized such that immediately after transmitting information in a slot, the transceiver adapts to receive information from a subsequent slot. This alternating transceiving procedure operates to allow continuous communication between two parties simultaneously.
TCM communication systems, however, suffer a detriment stemming from the fact that the recovered analog signal is discontinuous at the slot boundaries. In FIG. 1, a typical TCM signal recovery process 100 is shown. A time compressed signal is initially received in a series of time slots (102, 104, and 106). The information in each slot is expanded by the inverse of the compression ratio, and the signal is recovered by concatenating the information in a contiguous manner as shown (102', 104', and 106'). However, at the slot boundaries, discontinuities generally appear due to the limited bandwidth communication channel and imperfections in timing recovery at the receiver. These discontinuities cause distortion in the recovered signal As used herein, the boundary areas of the reconstructed slots is referred to as the "splice-zone". The "splice-zone distortion" 108 generally manifests itself in the reconstructed waveform as "pops" and "clicks" occurring at the TDM frame (slot) rate. To solve this problem, some designers have installed gain compression devices to lower the gain in the splice-zone to avoid amplifying the clicks and pops resulting from the distortion. Such gain compressors are both expensive and complicated since they must effectively reduce the gain in the splice-zone and restore it in a controlled fashion to properly amplify the intelligible information.
Generally, transceiving devices may be mobile units, portable units, or base stations. Generally, a portable unit is defined as a communication unit typically designed to be carried on or about the person. A mobile unit is a transceiving device designed to be installed in vehicles. A base station is contemplated to be a transceiving device permanently or semipermanently installed at a fixed location. As used herein, all of these devices are collectively referred to herein as transceiving devices.
Accordingly, a need exists for a spectrally efficient communication system capable of supplying increased information content down a limited bandwidth channel without introducing splice-zone distortion as in prior systems.